Apple is investing in a vast solar plant in Northern California that will generate as much electricity as the company uses to power all its operations in the state.

Apple will invest $850 million in the plant through a partnership with First Solar, CEO Tim Cook said Tuesday. It will cover 1,300 acres -- equal to about 1,000 football fields -- in Monterey County, about an hour south of Apples Silicon Valley headquarters.

The plant will generate enough energy that it could power Apples entire operations in California, including its data center, retail stores and offices. That's also enough energy to power 15,000 California homes, Cook said.

It doesn't mean Apple's stores and offices will consume power directly from the plant. But the investment allows Apple to lock in a low, fixed rate for renewable energy, and probably also obtain renewable energy certificates to offset its carbon foot print.

Apple will receive energy from 130 megawatts of the solar project under a 25-year purchase agreement, in the largest agreement of its kind to a commercial end user, First Solar said.

It's Apple's latest effort to improve its green credentials by investing in renewable energy; it already operates two solar farms on the east coast and one in Nevada. Google and other tech firms have made similar investments in renewables, though perhaps not on this scale.

"We're doing this because it's the right thing to do, but you may also be interested to know it's financially good to do it," Cook said at the Goldman Sachs Technology and Internet Conference. "We expect very significant savings."

Thats because the investment allows Apple to lock in a price for the renewable energy that will compare favorably to "brown energy," Cook said, meaning non-renewable sources like coal.

Construction is expected to begin in mid-2015 and be completed by the end of next year, First Solar said.